1. Field of the Invention
This invention generally relates to video disc players and more particularly is directed to a device for securely holding a video or other record disc during the rotation thereof.
2. Description of the Prior Art
Generally, in a video disc player, the information is recorded in substantially concentric circular tracks on a rotatable disc. The tracks may comprise successive turns of a continuous spiral or be separate, radially spaced circular tracks. When a high concentration of tracks is necessary to record an economically feasible amount of information on a single disc, it is desirable that the reproducing apparatus for reading information from the successive tracks be very accurately controlled and also that the alignment of the center of the video record disc in respect to the axis of rotation be held very accurately. When a frame of video information is recorded in each track, the video disc has to rotate at a speed of 1800 r.p.m. in the case of an NTSC signal or at a speed of 1500 r.p.m. in the case of a PAL signal. In order to insure that the disc alignment remains within operative limits at such speeds, the disc must be securely held or fastened to the rotary support therefor.
In an existing device for holding and rotating a video disc during reproduction of the informaton recorded thereon, the disc is held between a rotatable base and a clamping device. The clamping device is secured to the rotatable base by the mutual engagement of complementary threaded portions thereon. Force from the threaded engagement of such portions is transmitted directly through an annular rubber pad on the clamping device and a supporting surface on the rotatable base for clamping the record disc securely therebetween. A portion of the rotatble base engages in a circular opening at the center of the disc for providing accurate concentric alignment of the disc with the axis of rotation. Altnough the described device holds the record disc securely for accurate alignment during high speed rotation, changing or replacing the video disc requires that several turns of the clamping device be effected with respect to the rotatable base which operation is troublesome and undesirble for consumer applications.
Another mechanism for securing a record disc in accurate concentric alignment with a rotated support is disclosed in U.S. Pat. No. 3,609,722. Such mechanism includes a rotatable base with an upwardly facing supporting surface and an outwardly facing surface on an upstanding rim for engaging the central opening of the record disc to provide axial alignment therefor. The base further has a central boss projecting axially through the central opening of the record disc and having an inner circumferential recess. The mechanism further includes a clamping device having a cup-shaped body and an annular, radially directed flange slideably mounted thereon. A helical spring wound around the cup-shaped body biases the annular flange with respect thereto, so that when the cup-shped body is attached to the boss, the annular flange engages the upwardly facing surface of the record disc and the biasing force of the helical spring thereby exerts downward pressure on the top of the record disc. The cup-shaped body is releasably attachable to the boss by detent balls which project from a sleeve within the cup-shaped body and are engageable in the circumferential recess in central boss of the base. The detent balls are released in reponse to a downward pressure applied to push-button axially slidable in the sleeve. The application of such downward pressure on the push-button at the same time that the cup-shaped body is raised makes it difficult to remove the clamping device from the base.